This invention relates to a fuel injection timing device for adjusting the injection timing for fuel supplied from a fuel injection pump according to the operating conditions of an internal combustion engine.
In an engine of a fuel injection type, fuel is delivered from a fuel injection pump which is driven by the power of the engine, and the injection timing needs to be advanced or delayed according to the operation conditions of the engine, such as change of engine speed. Accordingly, a timer unit is interposed between the engine and the fuel injection pump so that the timing of the engine rotation is advanced or delayed by the timer unit, and is transmitted to a pump driving shaft, for example, a cam shaft of the pump.
FIG. 1 shows an arrangement of a conventional fuel injection timing device in which the fuel injection pump is located outside a cylinder block on the engine side and the timer unit is located between the pump and the cylinder block. In FIG. 1, a gear case 2 is coupled to a cylinder block 1 on the engine side by means of bolts 3. A fuel injection pump 4 is fixed to the gear case 2 by bolts 5. A timer unit 6 is housed in the gear case 2. The timer unit 6, which may be of any conventional type, e.g., of a hydraulically-operated or centrifugal-weight type, advances the timing of the rotation of the engine according to the operating conditions, and transmits the adjusted rotation to a cam shaft 7 used as a pump shaft. The timer unit 6 has a casing 8 and a driven gear 10 coupled to one end of the casing 8 by means of bolts 9. The driven gear 10 is in mesh with a driving gear 11 which is driven by the crank shaft of the engine. The cam shaft 7 is supported on a bearing cover 13 by a bearing 12, and the bearing cover 13 is attached to the pump 4 by means of bolts 14. The bearing cover 13 is fitted in one end of the gear case 2, the other end of which is fitted in an opening 1a of the cylinder block 1.
In the prior art construction as shown in FIG. 1, however, engagement between the driven gear 10 and the driving gear 11 is attained by centering between the cam shaft 7 and the opening 1a of the cylinder block 1 by successively mating the bearing cover 13, the gear case 2, and the opening 1a of the cylinder block 1 with one another. In this case, some fit tolerances need to be set to facilitate the assembly of the mating parts. These fit tolerances or erros, when added up, prevent accurate centering. As a result, the timer characteristic (engine speed-injection timing characteristic) of the timer unit is subject to hysteresis, so that the engine characteristics in case when the engine speed increases and decreases will vary differ from each other.
The gear case 2 is provided outside the casing 8 of the timer unit 6 to form a dual covering structure. Thus, both axial and diametrical dimensions of the timer unit 6 are substantially large, so that the timer unit 6 cannot be used if the space between the pump 4 and the cylinder block 1 is small.
Moreover, the dual structure requires a great distance L between the driven gear 10 and the pump housing 1. Therefore, a great bending moment will probably be applied to the cam shaft 7 to damage the same during the drive.